1. Field of the Invention
The present invention relates to printer apparatus and, more particularly, to a method of effecting linear travel of a lead screw driven carriage during the print cycle encompassed within a predetermined period of each stepped advancement thereof.
2. Description of the Prior Art
In lead screw driven matrix printers, a print head with a vertical column of either seven or nine selectively actuable print wires is mounted on a carriage and generally stepped across the width dimension of a print medium, such as paper in roll stock form. In the case of printing 5 .times. 7 dot matrix characters, the print head obviously must be stepped to five successive dot positions in order to form a print character within each character print column, with three dot spaces normally being employed to separate adjacent characters.
During each successive character column advancement of the print head, selected ones of the seven (or nine) wires are actuated or "fired" on-the-fly so as to drive the ends thereof either against an inked ribbon, and the latter against discrete portions of the paper, or directly against a pressure sensitive recording medium, to thereby effect the printing of a dot matrix character corresponding to the particular print wires actuated.
In such a matrix printer, the carriage is normally driven along a pair of guide rods aligned in parallel relationship with the lead screw. The carriage (and print head mounted thereon) is generally coupled to the lead screw by a threaded member, usually in the form of a drive nut, suitably mounted on the carriage. With the lead screw normally driven by a reversible stepping motor, for example, the rotational displacement of the lead screw is translated through the drive nut into linear displacement of the carriage (and print head). The direction in which the carriage is driven, and the speed of travel thereof, of course, is directly dependent on the direction and speed of rotation of the lead screw. For additional details of one preferred matrix printer of the type generally described hereinabove, and which is applicable for use in practicing the principles of the present invention, reference is made to a commonly assigned copending application of J. L. DeBoo-E. C. Feldy-H. S. Grear, Ser. No. 468,046, filed May 8, 1974, herein incorporated by reference.
While a power driven lead screw in printers of the dot matrix type affords a number of advantages over belts or chains for driving carriage-mounted print heads in terms of simplicity, ruggedness, cost and maximum possible driving speed, they nevertheless have presented a number of troublesome problems heretofore. Specifically, because of the necessity of threads, unless stringent tolerances are adhered to in the manufacture of the lead screw and drive nut, there must normally be either some backlash allowed for therebetween, or some form of a resilient, expandable drive nut employed in order to minimize the possibility of excessive frictional forces being established.
Various attempts to manufacture and mount the lead screw drive nut with stringent tolerances heretofore has proven to be impractical in practice for a number of reasons. First, a lead screw must necessarily extend across the entire width dimension of the printer, i.e., in parallel relationship with the platen and, as such, there is a tendency for the lead screw to inherently have or develop a slight bow which is most pronounced along the intermediate region thereof. Secondly, while the lead screw is normally mounted on precision ball bearings (or bushings), tolerance variations in the bearing mountings as manufactured, or as positioned on supporting frame structure of the printer, invariably leads to slight, but normally troublesome misalignment between the lead screw and carriage guide rods. Thirdly, because of the size of the threads and the axial length of the lead screw, a precision machining operation, as distinguished from a conventional and simple cold rolling operation, to form the threads would prove prohibitive from a cost standpoint.
Accordingly, even if a conventional drive nut could be manufactured to threadedly engage the lead screw in a very close fitting manner with negligible backlash, very high frictional forces would normally still develop not only between the lead screw and drive nut, but also between the lead screw and carriage guide rods. Such frictional forces would lead to excessive wear of the mating parts generating them, and could possibly overcome the driving torque of the stepping motor. In the latter case, the carriage would actually bind or lock-up on the guide rods. Such a problem, of course, could very possibly also seriously damage the drive mechanism in many printers.
Equally important, however, is the fact that any non-uniform frictional forces, whether great enough to actually bind the carriage or not, would necessarily at least alter the speed at which the carriage is either continuously driven or stepped along the guide rods. Such unintended variations in carriage speed during printing cannot be tolerated, as there must be a very precisely correlated relationship between the firing of the print wires (or hammers) and the lateral position of the print head at each successive dot position along a given print line.
In an attempt to solve some of the foregoing problems, specially constructed split nuts with garter springs and spring loaded "double" nuts have been tried, but both have been found to produce less than satisfactory results with respect to minimizing high frictional forces, excessive wear and/or distorted print characters due to non-linear carriage travel.
Thus, in order to reduce excessive frictional forces caused by tolerance variations, attempts have been made to intentionally construct the drive nut with a predetermined degree of backlash, or clearance, between the mating threads of a drive nut and lead screw. It becomes readily apparent, however, that whenever a built-in degree of backlash is employed in a lead screw-drive nut assembly, a substantial degree of kinetic energy is necessarily established by the mass of the coupled carriage, which includes the associated print head, during each advancement thereof. Such kinetic energy can, in turn, establish substantially large and detrimental impact forces between the lead screw and drive nut threads if not compensated for or absorbed in some way. These detrimental forces lead to a "bouncing" condition of the carriage (and print head).
One approach taken heretofore to absorb the abovedescribed type of kinetic energy imparted bounce forces has been to utilize a resilient, shock absorbing member between the drive nut and carriage. Such a member in one preferred form has comprised an O-ring which, in conjunction with mounting plates, has been further employed to resiliently mount the drive nut in a cantilevered manner on the carriage side wall. This allows the loosely coupled drive nut to acquire a slightly skewed condition relative to the axis of the lead screw, as may be required in order to compensate for inherent bow in the lead screw, and for any misalignment thereof relative to the carriage guide rods. To that end, one prior drive nut design has included both a threaded and an unthreaded bore section, the latter being oversized so as to facilitate radial displacement of the drive nut relative to the lead screw center line. For further details of several preferred embodiments of the above-described type of resiliently mounted drive nut and carriage assembly, reference is made to a commonly assigned copending application of A. F. Lindberg, Ser. No. 468,047, filed May 8, 1974, herein incorporated by reference.
Unquestionably, the above-described type of drive nut and carriage assembly has been found to provide substantial improvement over related prior assemblies in reducing wear, by simultaneously minimizing frictional forces and absorbing a substantial amount of the initial kinetic energy imparted carriage bounce force caused by backlash. However, the shock absorbing coupling member employed therein has been found in certain printers and applications to not always be capable of completely damping the initial impact bounce force. As a result, troublesome transient bounce forces may be generated in certain printers and continue for varying periods of time during the print cycle. This has been found to be particularly true whenever the purposely established backlash between the drive nut and lead screw is in the range of 0.02 to 0.04 inches, and the mass of the carriage and print head is greater than 10 ounces.
The presence of even minimal transient bounce forces, of course, can prove very detrimental, particularly in high speed dot matrix printers, wherein the carriage mounted print head is not only rapidly accelerated and decelerated in connection with each stepped advancement thereof, but has appreciable mass. As previously mentioned, any non-linear variation in the speed of carriage travel during the actual printing of the dots for each matrix character, for whatever reason, cannot be tolerated, as there must be a very precisely correlated relationship between the impact of the print wires upon the record medium and the lateral position of the print head at all times, if uniform dot spacings area to be realized.